1. Field of the Description
This invention relates to reacting a plurality of reactant gas streams in a matrix bed of heat-resistant matter. More particularly, this invention relates to increasing the volumetric reaction rate of the matrix beds.
2. Description of the Related Art
The prior art discloses reacting a plurality of reactant gas streams in a reactor having a matrix bed of heat-resistant material such that a planar reaction wave front is formed within the matrix bed. Examples of such reactors include stabilized reaction wave flameless thermal oxidizers and recuperative heating flameless thermal oxidizers, as disclosed in U.S. Pat. No. 5,320,518 to Stilger et al. entitled "Method and Apparatus for Recuperative Heating of Reactants in an Reaction Matrix" ("Stilger"), which is incorporated herein in its entirety by reference. In general, flameless thermal oxidizers operate by flamelessly thermally oxidizing gases within a porous matrix bed of heat-resistant material. The oxidation is called "flameless" because it may occur outside the normal premixed fuel/air flammability limits. Other examples and variations of flameless thermal oxidizers are disclosed in U.S. Pat. Nos. 4,688,495; 4,823,711; 5,165,884; 5,533,890; 5,601,790; 5,635,139; 5,637,283; and the U.S. patent application Ser. No. 08/659,579 entitled "Thermal Oxidizers with Improved Preheating Means and Processes for Operating Same," filed on Jun. 6, 1996, (Attorney Docket No. THER-0248), all of which are incorporated by reference herein in their entireties.
Prior Art FIG. 1 shows an example of a stabilized wave flameless thermal oxidizer. The oxidizer comprises a processor 10 having a matrix bed 11 of heat-resistant packing material supported at the bottom by a plenum 12 for distributing a mixture of a plurality of reactant gases 18 entering the matrix 11. The packing material may be comprised of ceramic balls, saddles, or ceramic foam of varying shapes and sizes or of other suitable heat-resistant packing. A void 13 over the top of the matrix 11 precedes an exit means 25 that penetrates the end wall 14 through which exhaust gases 22 exhaust. Through the bottom of the processor 10 is an inlet means 23 through which reactant gases 18 are introduced into the processor 10. The reactant gases 18 include control air, fuel, and process gas. If necessary, the fuel, air, or process gas may be heated prior to introduction to processor 10 by applying external heat to the mixed process gas prior to entering the processor 10. The plenum and lower portion of the matrix 11 may be heated by a suitable preheater 19 that, for example, may pass forced heated air into the processor 10, or heat the bed by electrical means. At various points in the matrix 11 are located temperature sensing devices such as thermocouples 20 from which the output is fed into a microprocessor or programmable logic controller 21 that, in turn, controls the proportions, volumetric flowrate, and temperature of the input gases entering the processor 10. The term "volumetric flowrate" shall be understood to refer to volumetric flowrate and/or mass flowrate.
Referring now to Prior Art FIG. 2, there is shown a schematic of the internal temperature zones and reaction wave front 22 of the stabilized reaction wave flameless thermal oxidizer. Typically, during operation, there will be a cool zone 27 below the uniform oxidation or combustion temperature that is being maintained within the reaction wave front. A planar reaction wave front 22 occurs in the matrix and has a stable shape with a radial, substantially uniform temperature distribution. Above the planar reaction wave front 22 will be a hot region 26. By using temperature sensors 20, the planar reaction wave front 22 may be relocated within the matrix by controlling the volumetric flows and conditions at the input end of the processor 10.
Referring now to Prior Art FIG. 3, a processor 80 of a recuperative heating flameless thermal oxidizer has an inlet port 88, an exhaust port 90, a heating port 92, a barrier 100, and a matrix bed 104. The inlet port 88 leads to an inlet plenum 94 at the bottom of the processor 80. A number of feed tubes 96 extend through an impermeable, rigid tubesheet 98 preferably made of steel or metal alloy, and a heat-resistant ceramic insulating barrier 100 at the roof of the plenum 94. The tubesheet 98 provides mechanical support for the tubes 96. The lower ends of the feed tubes 96 are provided with caps 102 to retain the matrix bed 104 inside the tubes 96. The caps 102 are provided with orifices 106 to permit the flow of gases from the inlet plenum 94 to the tubes 96. The matrix bed 104 is made up of heat-resistant packing material, as with the stabilized wave flameless thermal oxidizer, that is supported by the barrier 100. The packing material fills the region between the barrier 100 and the void 108 at the top of the processor 80 including the interior of the feed tubes 96. The matrix bed 104 may be heated by forcing heated gases, such as air, in through the heating port 92, and extracting the heated gases through the exhaust port 90. Alternatively, the bed may be heated by electric heaters or other means. During preheating, a low volumetric flow of ambient air may be bled through the inlet port 88 and up through the heat exchanger/feeding tubes 96 to ensure the tube material is not overheated, and to help establish the desired system temperature profile. Once the matrix bed 104 of the recuperative heating flameless thermal oxidizer has been preheated, the gases are introduced to the processor 80 through the inlet port 88. An adjusting means (not shown), that is analogous to the microprocessor or programmable logic controller 21 shown in Prior Art FIG. 1, also controls the volumetric flowrate and composition of the process gases to maintain a stable, planar reaction wave front that is similar to the planar reaction wave front 22 shown in Prior Art FIG. 2. Exhaust gases are extracted from the processor 80 through the exhaust port 90.
Now referring to Prior Art FIG. 4, a regenerative bed destruction system 210, an example of which is disclosed in U.S. Pat. No. 5,188,804 to Pace et al., entitled "Regenerative Bed Incinerator and Method of Operating Same" ("Pace"), and which is incorporated herein in its entirety by reference, may also be used to treat plurality of reactant gas streams 203. The destruction system 210 comprises a housing 212 enclosing a matrix bed 214, a lower gas plenum 216 disposed subadjacent the matrix bed 214, and an upper gas plenum 218 disposed superadjacent the matrix bed 214. Both the lower gas plenum 216 and the upper gas plenum 218 are provided with gas flow aperture openings 220 and 220', respectively. These openings 220 and 220' alternately serve as gas flow inlets or outlets depending upon the general direction of the flow of the reactant gas streams mixture through the matrix bed, which is periodically reversed as discussed hereinafter. A heating means 222, such as an electric resistance heating coil, is embedded within the central portion of the matrix bed 214. The heating means 222 is selectively energized to preheat the material in the central portion of the matrix bed 214 to a temperature sufficient to initiate and sustain a planar reaction wave front similar to the planar reaction wave front 22 shown in Prior Art FIG. 2.
During operation of the regenerative bed destruction system 210, the gas stream 203 flows into the bed 214 through either the lower gas plenum 216 or the upper gas plenum 216. The gas stream 203 flows through a supply duct 240 to a valve means 230. The valve means 230 receives the stream 203 through a first port 332 and selectively directs the received streams 203 through either the second port 234 or the third port 236. When the gas stream 203 is directed through the second port 234, the gas stream flows through duct 260 and opening 220 and into the lower plenum 216. When the gas stream 203 is directed through the third port 236, the gas stream flows through the duct 260' and opening 220' and into the upper plenum 218. The fourth port 238 of the valve means 230 is connected to the exhaust duct 270 through which the reactant product gas stream 205 is vented to the atmosphere. At spaced time intervals, the valve means 230 is actuated by controller 280 to reverse the flow of gases through the matrix bed 214. Every time that the flow is reversed, the role of the lower and upper gas plenums 216 and 218 is reversed with one going from serving as an inlet plenum to serving as an outlet plenum for the destruction system 210, while the other goes from serving as an outlet plenum to serving as an inlet plenum for the destruction system 210. In this manner, the upper and lower portions of the matrix bed alternately absorb heat from the reactant product gas stream leaving the central portion of the matrix bed from the shifting planar reaction wave front (not shown).
As previously noted, it is necessary to redirect the flow of gas stream 203 through the regenerative bed destruction system 210 to maintain a proper, planar, temperature profile within the matrix bed 214. Optimally, the planar temperature profile is hottest in the bed's center and cooler at its upstream and downstream edges. During proper operation, the reaction wave front migrates back and forth in the central portion of the matrix bed 214 in a direction parallel to the gas flow. If the gas flow direction is not properly switched, the reaction wave front will move out of the central portion of the matrix bed 214 and destroy the optimum temperature profile. To switch the gas flow direction, a controller means 280 activates the gas switching means 230 at timed intervals to reverse the direction of flow of the process exhaust gases. The controller means 280 also selectively activates the gas switching valve means 230 in response to the temperature of the reactant product gas stream 205. To this end, a temperature sensing means 290, such as a thermocouple, is disposed in the exhaust gas duct 270 at a location downstream of the gas switching valve means 230 for measuring the temperature of the reactant product gas stream 205. The temperature sensing means 290 generates a temperature signal 295 that is indicative of the temperature of the stream 205 leaving the downstream portion of the matrix bed 214, and transmits the temperature signal 295 to the controller means 280.
Other regenerative bed destruction systems may have multiple matrix beds, as is disclosed in U.S. Pat. Nos. 4,267,152 to Benedick entitled "Anti-Pollution Thermal Regeneration Apparatus" ("Benedick"); 3,895,918 to Mueller entitled "High Rate Thermal Regeneration Anti-Pollution System" ("Mueller"); 3,870,474 to Houston entitled "Regenerative Incinerator Systems for Waste Gases" ("Houston"); and 4,741,690 to Heed entitled "Process for Combustion or Decomposition of Pollutants and Equipment Therefor" ("Heed"), all of which are incorporated herein in their entireties by reference. In these systems (not shown), the plurality of reactant gas streams react in a first matrix bed, pass through an incinerator, and pass through a second matrix bed. The flow of the plurality of reactant gas streams is later reversed such that streams react in the second matrix bed, pass through the incinerator, and through the first matrix bed. As the gases react in the initial matrix bed through which they flow, they may or may not form a reaction wave.
These and other matrix bed reactor systems that form a reaction wave have an overall volumetric reaction rate limited by the area of the wave front. The overall volumetric reaction rate is the reactions occurring per matrix bed volume per time. The volumetric flowrates of the reactant gas streams are adjusted to establish and maintain the planar reaction wave front within the matrix bed. The overall volumetric reaction rate of the reactant gas streams cannot be raised by merely increasing the gas stream volumetric flowrates as this would push the planar reaction wave front out of the matrix bed, regardless of matrix bed length. To accommodate increased volumetric flowrates, the cross-sectional area of the matrix bed needs to be increased, thereby increasing the area of the planar reaction wave front.
However, simply increasing the area of the existing planar reaction wave front to accommodate increased reactant gas streams increases the size, and cost, of the matrix bed. Matrix bed reactor systems that generate planar reaction wave fronts have limits on their overall volumetric reaction rates based on their cross sectional areas. As a result, the volume of the matrix bed is dictated by the amount of reactions that will occur in the planar reaction wave front, preventing the design of a reduced-size matrix bed for applications with limited available space.
Thus, a need exists to provide a matrix bed with an increased overall volumetric reaction rate for reacting a plurality of reactant gas streams in a reaction wave front with the matrix bed having reduced fabricating costs and/or reduced space requirements.